In footwear, and specifically in athletic shoes, soles are typically comprised of several distinct layers, for example an outsole, midsole, and insole. The outsole is made of wear-resistant, tough material, and is patterned in such a way as to provide good traction and slip resistance. The outsole is typically in direct contact with the ground and the outermost layer of the footwear. The insole is designed for comfort and is typically made of soft cushioning material. By placing an appropriate midsole structure and/or material between the outsole and the insole, it is possible to provide a certain degree of shock absorption.
By way of one example, soles for walking shoes may be comprised of a flexible PVC outsole, a low density polyurethane insert for shock absorption, and a multilayered foot bed incorporating EVA, latex, and polyester (see for example U.S. Pat. No. 5,718,064).
Strategies for improving the shock absorptive properties of athletic footwear often include the use of compressible viscoelastic materials that are incorporated as insole or midsole elements under the foot or heel of the wearer. The purpose is to dissipate some of the impact energy when the heel strikes the ground during running or jumping.
A number of materials and designs have been used to address the need for improved shock absorption of midsoles. For example, open or closed cell elastomeric foams of different stiffness have been deployed in different areas of the midsole (see for example U.S. Pat. No. 4,614,046 and U.S. Pat. No. 4,364,188). These foams may be comprised of polymers such as polyurethane, polyethylene, or ethylene-vinyl acetate, also known as EVA.
An alternative is to insert air-filled bags within elastomeric foams (see U.S. Pat. No. 4,871,304). Other patents, (such as U.S. Pat. Nos. 4,535,553 and 5,343,639) teach midsoles that combine elastomeric foams with discretely spaced plastic projections, or a number of elastomeric foam columns between the upper and lower plates in the heel of the shoe, while the hollow spaces between the columns are filled with gas bladders.
Other patents teach the incorporation of gas filled bladders into soles (see U.S. Pat. Nos. 4,183,156, 4,219,945, 4,340,626, 4,936,029, 5,042,176 and 5,685,090) or a bladder composite comprised of an inner and outer bladder filled with cushioning or supporting fluids (see U.S. Pat. No. 5,979,078).
U.S. Pat. No. 5,915,819 teaches an adaptive, energy absorbing structure comprised of a plurality of fluid filled hexagonal cells joined together by passageways allowing fluid to intercommunicate. Pressure responsive seals are included that restrict the fluid flow between cells when a mechanical force exceeds a certain threshold level.
A similar concept is taught by U.S. Pat. No. 5,575,088 where concentric fluid filled toroids are contained in the midsole. To provide more stability and to address the problem of pronation during running that can lead to injury, some manufacturers of athletic shoes have incorporated midsoles that are less compressible and harder on the medial side of the heel midsole (see U.S. Pat. No. 4,614,046 and U.S. Pat. No. 4,364,188).
To provide better arch support, a molded shank can be integrated (see U.S. Pat. No. 6,061,929). Alternatively, springs (U.S. Pat. Nos. 5,042,175; 5,282,325; 5,381,608; 5,435,079; 5,743,028; 6,055,747, and U.S. Publication No. 2015/0013191) or resilient materials (see U.S. Pat. Nos. 5,092,060; 5,311,674) can be incorporated that are intended to not only provide cushioning, but also return some energy.
However, such energy return can contribute to enhanced shock to an athlete. The spring action within the heel of a shoe can to some extent be adjusted by inserting foam rubber inserts of varying density (see U.S. Pat. No. 5,544,431). To reduce the amount of detrimental energy return, telescopic shock absorbers have been incorporated (see U.S. Pat. No. 6,457,261).
The shock absorbers can have two stages with different compression levels, namely a first stage that provides cushioning at low levels of load, such as walking, and a second stage that is able to absorb the higher loads from activities such as jumping or running.
Typical impact energy absorbing materials used in footwear are open or closed cell foams of various thermoplastic polymers including polyurethane, polyethylene, polystyrene, as well as foams or dense bodies of elastomeric polymers, including silicones, ethylene vinyl acetate (EVA), ethylene-propylene rubbers (EPM), ethylene-propylene-diene rubbers (EPDM). In addition to single component materials, various composite materials have been reported. Many of these materials contain mixtures of polyethylene with fibers (see U.S. Pat. No. 4,946,721). Other approaches include composites of rigid hollow spheres encapsulated in an elastomeric matrix (see U.S. Pat. No. 4,101,704), or composites of elastomers with fillers (see U.S. Pat. No. 4,082,888).
There have also been attempts to improve the impact energy absorption capacity of materials by laminating different layers together. For example, European Patent EP 0 955 211 B1 teaches impact energy absorbing materials for protective athletic gear using layers of expanded polytetrafluoroethylene (ePTFE) and at least one layer of an elastomer. U.S. Pat. No. 6,023,859 teaches an insertable member of the midsole. U.S. Pat. No. 6,205,681 teaches the use of a midsole that is formed from a soft elastic material and a corrugated sheet in the heel portion. The patent claims that the cushioning properties of the shoe can be improved by introducing holes in the midsole at locations where the midsole contacts the corrugated sheet, thereby facilitating vertical deformation. U.S. Pat. No. 8,453,344 describes a sole that has a reinforcing member in the midsole comprised of several interconnecting blades. U.S. Pat. No. 8,973,287 describes a sole plate that has a number of blades that are standing on it vertically. The sole plate is bonded to a cover, and a fluid is sealed in between the sole plate and the cover. The purpose of the fluid is to provide movement during walking that massages the foot with the blades. U.S. Publication No. 2015/0143713 describes a multi-function shoe pad that includes a hollow bulge forming an air filled chamber that is deformable between a compressed and uncompressed state, so that hot air can move out and cold air can move in to provide cooling. U.S. Publication No. 2015/0157091 teaches a shock absorbing and pressure releasing damper apparatus.
Based on the brief description of the selected prior art above, it is clear that most ways current footwear manufacturers attempt dissipate energy is to use pliable, relatively soft materials. The underlying theory for this design is the idea that soft materials should be able to cushion against impact. These materials, however, because of their composition, shape and orientation in the footwear are designed to only vertically compress, which permits a large portion of the impact energy to be transferred to the foot.
In view of disadvantages of the prior art design, it would be advantageous to use a relatively hard polymeric material that has lower compressibility compared to conventional insole materials. As such, the insoles are better able to convert the kinetic energy during an impact into heat and sound and reversible deformations of the boundaries of open spaces in the material, thereby lessening the remaining forces transmitted to the heel or forefoot.